The invention concerns an arrangement for attaching blades to the wheel of a rotor.
A blade attachment for axial-flow gyrowheel machines is known from German utility model 1739025, in which the blades are kept interlocked by spiral-shaped grooves around the circumference of the wheel and on the feet of the blades. The foot of each blade has a threaded section, and there is a threaded section on the foot of the adjacent blade with a continuous thread that matches the thread on the circumference of the wheel. The pitch of the spirals is chosen so that the connection is self-locking.
The known connection is sufficient for rotors subject to low stresses. For highly stressed rotors of turbines or compressors with peripheral speeds up to 500 m/sec, however, tractive forces up to 2 tonnes can occur in the blade, in addition to thermal stresses. On the other hand, at wheel diameters of approximately 15 cm, the only surfaces available for transmitting the forces are small, i.e., when the known groove connection is used, at least three thread turns must mesh, which is hard to achieve with a thread with only one turn. But threads with many turns, because of their higher pitch, reach their self-locking limit faster.
Moreover, the known connection does not take into consideration the fact that because of the asymmetry of the spiral thread, a rotor without blades can be unbalanced. At the high stresses specified, imbalances must be avoided at all costs.
The task of the invention is to design an attachment arrangement of the type specified at the beginning to make sure that the blades will not come loose during operation, and that there is no inherent design imbalance.
To solve the task, the present invention provides: that the thread on the wheel is designed to have one turn; that the start of the thread on one side of the wheel is slightly staggered, compared to the start of the thread on the other side, and runs in the opposite direction around the circumference; and that the threaded prestressed section of the blade foot meshes with the threads on the wheel. The eccentricity of one spiral thread is thus offset by the opposite thread. The initial stress between the blade feet and the wheel also guarantees that the blades fit securely during all stresses that may occur.
Both threads on the wheel can be designed as outer threads on the outer edge on both opposite sides. In this way, the mass is kept small, and the wheel can be produced with no cutting, for example, by roll forming, which substantially increases the permitted stress. The corresponding inner threads are provided on the blade feet in a slot which extends radially from the inside to the outside in the web forming the foot.
Since the foot of each blade has only a short section of the total thread, both opposing threads are relatively simple to produce.
However, it is likewise possible to give the outer edge of the wheel a groove that runs in the direction of the circumference and extends radially to the inside, and which has an inner thread on each of the opposite inner flanks, while the outer threads have webs forming the blade foot on both sides.
The screwed-in blades, under the initial stress, are preferably fixed by means of stop faces, which can be designed on the lower edge of the blade foot or on flanges on the transition to the blade profile. The initial stress is preferably produced thermally, by cooling the blades and turning them when cold with a minimal expenditure of force, with almost no friction in the direction of the circumference on the wheel until the first and last blade stand next to one another with no play. Subsequent reheating to room temperature then produces the desired initial stress. Alternatively, or even additionally, the wheel can be heated before the blades are put on.
When the blades are connected to a cover band, or so-called "clappers", the blade attachment in the invention also makes it possible for them to be attached to the cover band or clapper, wherein the individual cover band or clapper elements lie close to one another with their edges touching for this purpose. Thus, initial stress can also be produced in the cover band, which counteracts the stresses that occur during operation. For example, the blades can also be screwed in without stress by cooling the cover band on the ends of the blades, so that when the ends of the blades are warmed to room temperature, a compressive strain is produced in the cover band. The result of this is a tractive force in the blade and a tangential tractive force in the wheel. One advantageous consequence is that a smaller sealing gap is provided so that, for example, the efficiency of the individual rotating blades of a drive gear can be increased considerably.
In order that the wheel is able to take higher tangential stresses, it can have a winding of fibrous materials, which are ideally stressed in their tractive direction, along its circumference. This winding can be in a rotating slot, which is locked by the foot of the blade. This has the simultaneous advantage of protecting the winding from outside influences. The slot is advantageously locked with an adapter, which at the same time prevents the slot flange from bending during stress. This adapter is conveniently formed by one part of the foot of the blade.
The fibers used can be, for example, fiberglass, carbon fibers, silicon carbide fibers or metallic fibers, or combinations thereof. It is essential that the fibers have a high tensile strength and a low density. For the same stress, the wheel can be built substantially lighter. To achieve initial stress in the winding, the wheel body can be cooled before and during the attachment of the winding. After an almost tension-free attachment of the winding, and subsequent heating of the wheel to room temperature, there is a tangential tractive initial stress in the winding, and a tangential compressive strain in the remaining wheel body. The winding can be applied without an embedded matrix, so that the material properties can be used to the utmost, especially at high fiber temperatures.
By choosing a suitable fiber, the life of a wheel can be lengthened considerably. The wheel filled with the fiber materials can be stopped at an initial compressive strain, while during operation the compressive strain is gradually decreased and transferred to a tractive force, i.e., the tension amplitude remains, but the median value of the stress falls to a lower level.
When using a winding from fibers subject to a tractive force, the slot enclosing the winding may have flanges pointing inward in the cross section on the outer periphery of the wheel, which overlap the blade foot from the outside. The blade foot can then be designed as a so-called "hammer foot", wherein one thread can be eliminated, if necessary.
Another improvement in the efficiency of the winding is possible if the winding is composed of different fibers. Thus, in the radial inner area of the winding, fibers with a large modulus of elasticity (e.g., soft fibers) can be used, making it possible for a force to be introduced in the hard fibers lying on top.
The blades are further secured in the wheel when the thread flanks are inclined at an angle outward on the blade feet in the radial direction. The blades will then be pressed on the wheel to interlock non-positively by the effect of centrifugal force.
Furthermore, the spacers can be arranged between the blade feet so that the distance between individual blades is increased somewhat, producing at the same time an increase in the radius, so that the sealing gap in the area of the cover band can be reduced or set. In this way, during repairs or when the cover band wears out, the sealing gap can be reset exactly. The spacers need not be used between all of the blade feet, but can be adjusted in number according to the respective requirements. It is important only that they are distributed evenly over the periphery of the wheel so that there is no imbalance. They can be designed as self-supporting clamping rings, which are curved upward and inserted between two blade feet.